The present invention relates in general to video projection systems, and more particularly to an apparatus for increasing the efficiency of light collection from a lamp into the integrating rod of a video projector.
The standard illumination configuration of a D-cinema video projector includes an arc lamp and reflector for reflecting light to a focal point. An SXGA aspect ratio integrating rod is positioned at the focal point to collect and integrate the light. Typically, a 7 KW Xe lamp and reflector are used to focus the light at f/1.5. Research using optical traces has shown that considerable light is lost at the input aperture to the rod when using long arc lamps, due to the large spot size at the focal point (e.g. up to 25.4-mm diameter). The resultant overfilling of the integrator rod reduces the collection efficiency at the exit face (e.g. down to 28%). This, in turn, results in reduced brightness for the projection system.
The present invention provides a new method and mechanism of light collection that compresses the focal spot produced by the reflector/lamp combination to increase the amount of light entering the integrator rod with no consequential increase in numerical aperture of the optical system. More particularly, a converging lens is introduced between the reflector/lamp and the input aperture of the integrating rod for compressing the focal spot produced by the lamp and thereby increasing the input energy into the integrating rod. As a result of the principle of optical invariance, the decreased magnification from the converging lens also decreases the f/#. Therefore, according to another aspect of the invention the focal spot size is increased in the integrator rod to restore the required f/#.
According to a preferred embodiment of the invention, a diverging lens is attached to the integrating rod for restoring the f/#, although it is contemplated that gradient index glass or other optical mechanisms may be used instead.
The optical arrangement according to the present invention results in greater light collection efficiency than provided by existing projector systems without compromising optical invariance of the system.